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LTC2851 Datasheet(PDF) 11 Page - Linear Technology |
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LTC2851 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 18 page  LTC2850/LTC2851/LTC2852 285012fd applicaTions inForMaTion Driver ThedriverprovidesfullRS485/RS422compatibility.When enabled, if DI is high, Y-Z is positive for the full-duplex devices (LTC2851, LTC2852) and A-B is positive for the half-duplex device (LTC2850). When the driver is disabled, both outputs are high im- pedance. For the full-duplex devices, the leakage on the driver output pins is guaranteed to be less than 10µA over the entire common mode range of –7V to 12V. On the half-duplex LTC2850, the impedance is dominated by the receiver input resistance, RIN. Driver Overvoltage and Overcurrent Protection The driver outputs are protected from short-circuits to any voltage within the Absolute Maximum range of (VCC – 15V) to 15V. The typical peak current in this condi- tion does not exceed 180mA. If a high driver output is shorted to a voltage just above VCC, a reverse current will flow into the supply. When this voltage exceeds VCC by about 1.4V, the reverse current turns off. Preventing the driver from turning off with outputs shorted to output voltages just above VCC keeps the driver active even for receiver loads that have a positive common mode with respect to the driver— a valid condition. The worst-case peak reverse short-circuit current can be as high as 300mA in extreme cold conditions. If this current can not be absorbed by the supply, a 3.6V Zener diode can be added in parallel with the supply to sink this current. All devices also feature thermal shutdown protection that disables the driver and receiver in case of excessive power dissipation (see Note 4 in the Electrical Characteristics section). Receiver and Failsafe With the receiver enabled, when the absolute value of the differential voltage between the A and B pins is greater than 200mV, the state of RO will reflect the polar- ity of (A-B) These parts have a failsafe feature that guarantees the receiver output to be in a logic-high state when the inputs are either shorted, left open, or terminated but not driven. This failsafe feature is guaranteed to work for inputs spanning the entire common mode range of –7V to 12V. The receiver output is internally driven high (to VCC) or low (to ground) with no external pull-up needed. When the receiver is disabled the RO pin becomes Hi-Z with leakage of less than ±1µA for voltages within the supply range. Receiver Input Resistance The receiver input resistance from A or B to ground is guaranteed to be greater than 96k (C, I-grade). This is 8x higher than the requirements for the RS485 standard and thus this receiver represents a one-eighth unit load. This, in turn, means that 8x the standard number of receivers, or 256 total, can be connected to a line without loading it beyond what is specified in the RS485 standard. The receiver input resistance from A or B to ground on high temperature H-grade parts is greater than 48k providing a one-quarter unit load. The high input resistance of the receiver is maintained whether it is enabled or disabled, powered or unpowered. Supply Current The unloaded static supply currents in these devices are very low, typically under 500µA for all modes of opera- tion. In applications with resistively terminated cables, the supply current is dominated by the driver load. For example, when using two 120 Ω terminators with a dif- ferential driver output voltage of 2V, the DC load current is 33mA, which is sourced by the positive voltage supply. Power supply current increases with toggling data due to capacitive loading and this term can increase significantly at high data rates. Figure 13 shows supply current vs data rate for two different capacitive loads for the circuit configuration of Figure 4. |
Similar Part No. - LTC2851_15 |
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Similar Description - LTC2851_15 |
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